A Freestanding Chitin-Derived Hierarchical Nanocomposite for Developing Electrodes in Future Supercapacitor Industry

Crustacean cuticles are receiving extensive attention for its potential in developing environmentally friendly and high energy density electrodes for supercapacitor applications. In the current work, the demineralized tergite cuticle of mantis shrimp was employed as a precursor for the fabrication porous biochar. The structural benefits of the cuticle, including the hierarchical nanofiber networks, and the interpenetrating pore systems were maximumly retained, providing a high carbon content and specific surface area scaffold. Graphene oxide sheets were deposited across the biochar through the pore canal systems to further increase the conductivity of the biochar, forming a novel freestanding carbon composite. Throughout the modification process, the material products were examined by a range of methods, which showed desired structural, chemical and functional properties. Our work demonstrates that high performance carbon materials can be manufactured using a simple and green process to realize the great potential in energy storage applications.


Introduction
Life in today's world is highly dependent on energy. A vast amount of energy is generated across the world by using traditional fossil fuels. As a direct consequence of the consumption of fossil energy having a devasting impact on the environment and global warming, renewable energy sources, such as solar and wind energy, are increasingly accounting for a significant proportion of the overall energy powering our communities [1,2]. At the same time, the development of powerful energy storage materials for these emerging energy systems, which mainly depend on variable renewable resources, is becoming urgent. Among the many energy storage materials available in the marketplace, batteries are undoubtedly dominating the market due to its superior energy density [3]. However, the fatigue of voltage capacity over time is a key issue, and most of the batteries are generally harmful to the environment and may cause safety risks. Rather than relying on chemical reactions in order to function, capacitors manage to store potential energy electrostatically instead. Capacitors, therefore, present the potential to be an alternative to the storage material community, but they are not yet a substitute for batteries, mainly due to the low energy density. In comparison with the traditional capacitors, such as EDLC and pseudocapacitors, the distances between the two metal plates of supercapacitors are much smaller, resulting in a superior power throughput, and the higher specific surface area in supercapacitors In this study, by using the crustacean cuticle as a 3D scaffold and taking advantage of its interpenetrating and porous structure characteristics, we successfully transformed it into a novel self-supported and highly conductive carbon material through straightforward chemical modification processes ( Figure 1). The encased mineral components (principally amorphous calcium carbonate (ACC) and a small amount of amorphous calcium phosphate (ACP) and calcite) in mantis shrimp cuticle were firstly removed by HCl. The products were then immersed in graphene oxide (GO) aqueous solution until the samples were sufficiently encased with GO sheets. After an oven-drying process, the samples were then heated to around 800 °C in a Nitrogen atmosphere, which expelled most of the noncarbon atom within the demineralized cuticle, along with turning the deposited GO into reduced GO (rGO), forming a freestanding composite which retained most of the intrinsic 3D structural properties of the original cuticle and with a high degree of graphitization. The developed material shows excellent performance in conductivity and could easily be manufactured. This would create great potential for developing advanced supercapacitors, strain sensors and battery materials.

Sample Preparation
The tergites of mantis shrimp (Odontodactylus scyllarus) were used as precursors for the preparation of nano-porous carbons. The shrimps were obtained from online retailers in Taobao and refrigerated at −80 °C after dissecting. Before demineralization, the cuticle dissected from defrosted specimens were washed thoroughly, cleaned with deionized water to remove impurities, and then dried in a hot air oven at 50 °C for 3 h.

Demineralization Protocols
Demineralization was carried out by immersing the dried cuticle samples in 1 M HCl with agitation speed of 400 rpm for 3 h at room temperature [32][33][34]. The demineralized cuticles were then immersed in deionized water for 30 min, with constant stirring until In this study, by using the crustacean cuticle as a 3D scaffold and taking advantage of its interpenetrating and porous structure characteristics, we successfully transformed it into a novel self-supported and highly conductive carbon material through straightforward chemical modification processes ( Figure 1). The encased mineral components (principally amorphous calcium carbonate (ACC) and a small amount of amorphous calcium phosphate (ACP) and calcite) in mantis shrimp cuticle were firstly removed by HCl. The products were then immersed in graphene oxide (GO) aqueous solution until the samples were sufficiently encased with GO sheets. After an oven-drying process, the samples were then heated to around 800 • C in a Nitrogen atmosphere, which expelled most of the non-carbon atom within the demineralized cuticle, along with turning the deposited GO into reduced GO (rGO), forming a freestanding composite which retained most of the intrinsic 3D structural properties of the original cuticle and with a high degree of graphitization. The developed material shows excellent performance in conductivity and could easily be manufactured. This would create great potential for developing advanced supercapacitors, strain sensors and battery materials.

Sample Preparation
The tergites of mantis shrimp (Odontodactylus scyllarus) were used as precursors for the preparation of nano-porous carbons. The shrimps were obtained from online retailers in Taobao and refrigerated at −80 • C after dissecting. Before demineralization, the cuticle dissected from defrosted specimens were washed thoroughly, cleaned with deionized water to remove impurities, and then dried in a hot air oven at 50 • C for 3 h.

Demineralization Protocols
Demineralization was carried out by immersing the dried cuticle samples in 1 M HCl with agitation speed of 400 rpm for 3 h at room temperature [32][33][34]. The demineralized cuticles were then immersed in deionized water for 30 min, with constant stirring until Polymers 2022, 14, 195 4 of 14 reaching neutral pH, and dried again in an oven. Finally, the demineralized cuticles were kept in an air-tight container for further use.

Preparation of Graphene Oxide (GO)
GO was prepared following the steps described in the ingeniously modified Hummer's method [35][36][37]. The expanded graphite (0.5 g) was added to 22 mL of concentrated H 2 SO 4 with agitation at 0 • C for 5 min. KMnO 4 (1.5 g) was then slowly added to ensure the temperature of the mixture was kept well below 20 • C. Following this, the obtained solution was further stirred for 5 h at a temperature range of 10 to 15 • C. The mixture was then heated to 30 • C and stirred for another 30 min. After that, 24 mL of deionized water was slowly added into the solution and further stirred for 30 min at 80 • C. The reaction was terminated by adding 4 mL of H 2 O 2 to remove the excess KMnO 4 . The obtained solution, after adding 30 mL H 2 O, was centrifuged at 4000 rpm for 10 min. Subsequently, the obtained deposition was washed by stirring 200 mL of 5% HCl under a water bath at 40 • C. The deposition was subsequently washed by deionized water until the pH is close to neutral. To facilitate the permeation of GO into the demineralized cuticles, the depositions was ultrasonically crushed in water for 30 min; then, the suspension was centrifuged and lyophilized to obtain GO flakes. The dimension of the GO flakes ranged from 10 to 300 nm determined by atomic force microscopy (AFM). The GO flakes were stored in an air-tight container placed at 4 • C in refrigerator.

Carbonization
The demineralized cuticles were firstly immersed in 5 mg/mL graphene oxide (GO) aqueous solution with gentle shaking at room temperature for 2 days [38,39].The GO impregnated samples were briefly rinsed with distilled water and then dried in an oven at 50 • C for 3 h. Further, the samples were transferred into tube furnace and received high temperature pyrolysis under a constant N 2 flow, with a heating rate of 5 • C/min up to 800 • C and held for 2 h [31]. After the pyrolysis, the chitin-protein nanofiber (CF) and the GO-deposited chitin-protein nanofiber (CF/GO) turned into carbonized chitin nanofiber (CCF) and carbonized chitin nanofiber with reduced graphene oxide (CCF/rGO), respectively. All the generated CCF and CCF/rGO were taken out and kept in an air-tight container after cooling down to the room temperature.

Material Characterization
The morphological and topographical features of the samples were observed by scanning electron microscopy (SEM, ZEISS Gemini 300, Oberkochen, Germany). This was performed in the secondary electron mode at the acceleration voltage of 3 kV. Nano-CT characterization with volumetric spatial resolution around 64 nm was carried out by using a full-field transmission hard X-ray microscopy system installed at the beamline 4W1A in Beijing Synchrotron Radiation Facility (BSRF, Beijing, China) with a beam energy of 8 keV. Segmentation was conducted with software Avizo. The X-ray diffraction (XRD) measurements were conducted at the beamline 3W1 by using a 2 mm beam (16 keV) at BSRF. An Eigger 1 M detector (DECTRIS AG, Baden-Daettwil, Switzerland) with a pixel size of 75 µm and a resolution of 1030 × 1065 pixels (horizontal and vertical) was used to record the XRD patterns. The natural surface of the cuticle was oriented perpendicular to the X-ray beam. The sample-to-detector distance (about 81.1 mm) was calibrated using silver behenate (AgBe). Small-Angle X-ray Scattering (SAXS) tests were performed at the Timeresolved Ultra Small-Angle X-ray Scattering (USAXS) beamline with a monochromatic beam (beam size: 10 × 10 µm 2 ) of 10 keV at the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The diffraction patterns were collected using a PILATUS 1M detector (DECTRIS AG, Baden-Daettwil, Switzerland) with pixel size of 172 µm and sampleto-detector distance of 3.11 m under exposure time at 1 s. Atomic force microscopy (AFM, Dimension-Edge, Bruker, Berlin, Germany) tests were operated in the tapping mode. Thermogravimetric analysis (TGA, TA TGA55, TA, New Castle, DE, USA) was performed in N 2 atmosphere from 30 to 800 • C with heating rate of 5 • C/min. N 2 adsorption experiments (ASAP 2460 Version 3.01, Micromeritics, Norcross, GA, USA) were conducted at −196 • C after the samples were degassed under vacuum at 200 • C for 6 h. The specific surface area of the samples was measured by using Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method based on adsorption isotherm. X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, St Louis, MO, USA) was carried out using an Al Kα X-ray source with beam size of 400 µm. The electrical conductivity of the materials was measure using a four-point probe system. The lyophilized process of GO was conducted by a vacuum freeze dryer (Biocool, FD-1A-50, Beijing, China) at −50 • C. FTIR tests were conducted on an infrared spectrometer (Thermo Scientific Nicolet iS20, St Louis, MO, USA). A total of 16 scans were performed over the range of 4000-550 cm −1 at a step size of 4 cm −1 under transmittance mode. Raman spectrum were conducted using a Raman spectrometer (Renishaw inVia Raman, Gloucestershire, UK) with a laser wavelength of 473 nm.

D structural Characterization of Mantis Shrimp Cuticle
The exoskeleton of crustacean is a multiscale biological material, which is composed by of a chitin-based fibrillar network and reinforced by the incorporation of biomineral particles. These chitin fibrils form a characteristic rotated layered plywood (Bouligand) structure at the scale of~10 µm, which develops into a well-defined honeycomb lattice-like system with pore canal network running perpendicular to these lamellae [19]. The cuticle of mantis shrimp is a prototype of crustacean exoskeleton with interpenetrating pore canal network ( Figure 2a). Nano-CT (3D spatial resolution of 64 nm) is employed to reveal the detailed microstructure of the cuticle for better understanding of the pore canal network. A well-developed 3D pore canal system imaged by nano-CT is presented in Figure 2b, showing a highly connected pore canal system ( Figure 2eI). Most of the crustaceans harden the cuticle, not only by sclerotization but also by calcification [40,41], which means they need energetically demanding calcium turnover after ecdysis. The 3D pore canal network serves as channels for material transport during molting, and highly interpenetrating pore canals ensure efficient turnover of calcium. Additionally, as shown in Figure 2c,d, the pore canal contains a long and firm tube with a diameter of about 1 µm, which is reinforced by mineral particles (Figure 2eII). The highly interpenetrating pore canal network makes the cuticle an ideal precursor to prepare hierarchical carbon composites.

Morphology of Composites
Macroscopically, the pristine cuticle consists of four layers: epicuticle (epi), exocuticle (exo), endocuticle (endo) and membranous (mL) layers ( Figure 3a) [19]. Epicuticle is a permeability barrier to the environment which is thin and waxy. As the epicuticle consists mainly of esters of fatty acids and calcite, and no chitin [42], it should be removed after demineralization [34]. Both the exocuticle and endocuticle layers are formed by parallel arrays of mineralized chitin fibers (in-plane fibers, IP), which mainly contribute to the mechanical properties of the procuticle. As shown in Figure 3d, in these two layers, the IP are arranged into a twisted plywood known as Bouligand, which is interpenetrated with the perpendicularly oriented out-of-plane fibers (OP). After demineralization, the chitin molecules were still wrapped by proteins in shells and are responsible for the stability of chitin-protein nanofibers (CF). Eventually, the CF scaffold shows only two layers (Figure 3b): exocuticle and endocuticle. In addition, the twisted plywood ( Figure 3e) and honeycomblike structure (Figure 3b: insert) of mineral chitin fibers with in native cuticle substantially unchanged in those layers after demineralization. After functional with GO, the gaps between the fibers are filled with GO (Figure 3c,f). This excellent coverage is achieved due to strong attractive forces, such as van der Waals and hydrogen bonding between polar oxygen-containing groups of chitin and GO.

Morphology of Composites
Macroscopically, the pristine cuticle consists of four layers: epicuticle (epi), exocuticle (exo), endocuticle (endo) and membranous (mL) layers (Figure 3a) [19]. Epicuticle is a permeability barrier to the environment which is thin and waxy. As the epicuticle consists mainly of esters of fatty acids and calcite, and no chitin [42], it should be removed after demineralization [34]. Both the exocuticle and endocuticle layers are formed by parallel arrays of mineralized chitin fibers (in-plane fibers, IP), which mainly contribute to the mechanical properties of the procuticle. As shown in Figure 3d, in these two layers, the IP are arranged into a twisted plywood known as Bouligand, which is interpenetrated with the perpendicularly oriented out-of-plane fibers (OP). After demineralization, the chitin molecules were still wrapped by proteins in shells and are responsible for the stability of chitin-protein nanofibers (CF). Eventually, the CF scaffold shows only two layers ( Figure  3b): exocuticle and endocuticle. In addition, the twisted plywood (Figure 3e) and CF and GO functional chitin-protein fiber (CF/GO) were used as robust frameworks for the composite synthesis. The pyrolysis temperature has an important influence on the porous structure and graphitic structure of biomass [31]. Increasing the pyrolysis temperature within a certain range will cause the number of pores (especially micropores) and the specific surface area of biomass to increase, which is beneficial to the formation of a good porous structure. At the same time, when the pyrolysis temperature increases, the graphitic structure of biomass will gradually develop, and the whole crystal structure tends to be orderly, resulting in an increase in the degree of graphitization. Yu et al. gained porous graphitic carbon (PSS-bio) by pyrolysis of shrimp shells at different temperatures [31]. They detected that pyrolysis temperature is the key element to regulate the porous structure and carbon configuration of PSS-bio. Based on their conclusion, the pyrolysis condition was set at a heating rate of 5 • C/min up to 800 • C and held for 2 h. With the treatment of high temperature, the precursors simultaneously triggered the transformation of precursor into self-supported electrode materials: carbonization chitin-protein nanofiber (via carbonization) and GO into rGO (via reduction). The ensuing materials are freestanding nano-structured carbon matrixes with 20 µm thickness. Interestingly, porous and the lamellar structure of CF scaffold can be well preserved after high-temperature treatment, and the only difference is that the lamellae spacing became smaller, which can be confirmed by SEM and XRD characterization, as shown in Figures 4 and 5. The micropore-canal networks of CCF and CCF/rGO, respectively, shown in Figure 4a,b, serve as ion channels, which enhance the quick penetration of electrolyte ions into the electrode material [13,16]. The CCF/rGO demonstrate a completely different structure in contrast to CCF. The pore canal network of CCF/rGO was protected with two thin carbon frames, as shown in Figure 5b, which might improve fatigue resistant of carbon material and mechanical stability. With the protection of a thin tablet, stress can be dissipated into the whole pore canal network. Generally, this lamellar structure has a dramatic effect on fracture toughness [43]. The cracks prefer to propagate along the adjacent layers, since propagation across different lamellar layers would need higher stress to tear the fiber sheets.
the specific surface area of biomass to increase, which is beneficial to the formation of a good porous structure. At the same time, when the pyrolysis temperature increases, the graphitic structure of biomass will gradually develop, and the whole crystal structure tends to be orderly, resulting in an increase in the degree of graphitization. Yu et al. gained porous graphitic carbon (PSS-bio) by pyrolysis of shrimp shells at different temperatures [31]. They detected that pyrolysis temperature is the key element to regulate the porous structure and carbon configuration of PSS-bio. Based on their conclusion, the pyrolysis condition was set at a heating rate of 5 °C/min up to 800 °C and held for 2 h. With the treatment of high temperature, the precursors simultaneously triggered the transformation of precursor into self-supported electrode materials: carbonization chitin-protein nanofiber (via carbonization) and GO into rGO (via reduction). The ensuing materials are freestanding nano-structured carbon matrixes with 20 μm thickness. Interestingly, porous and the lamellar structure of CF scaffold can be well preserved after high-temperature treatment, and the only difference is that the lamellae spacing became smaller, which can be confirmed by SEM and XRD characterization, as shown in Figures 4 and 5. The micropore-canal networks of CCF and CCF/rGO, respectively, shown in Figure 4a,b, serve as ion channels, which enhance the quick penetration of electrolyte ions into the electrode material [13,16]. The CCF/rGO demonstrate a completely different structure in contrast to CCF. The pore canal network of CCF/rGO was protected with two thin carbon frames, as shown in Figure 5b, which might improve fatigue resistant of carbon material and mechanical stability. With the protection of a thin tablet, stress can be dissipated into the whole pore canal network. Generally, this lamellar structure has a dramatic effect on fracture toughness [43]. The cracks prefer to propagate along the adjacent layers, since propagation across different lamellar layers would need higher stress to tear the fiber sheets.

Structure Characterization
SAXS and XRD are applied to probe the structure changes of the products after demineralization and carbonization. Figure 5a shows the XRD results of cuticles treated with HCl for different hours. As may be seen from the corresponding 1D intensity profile I(Q) curves (Figure 5a), the lattice spacing of chitin D (110) and (013) in demineralized cuticle is not significantly different from the native cuticle, while the peaks from mineral phases vanished after treating with HCl for 1 h. Even after a longer time of HCl treatment (3 h and 18 h), the lattice spacing of chitin still was not significantly different from the native cuticle. The SAXS pattern of native cuticle (Figure 5c) shows a circular scattering pattern, and the scattering intensity mainly came from the ACC and ACP within the cuticle. While the scattering intensity of CF (Figure 4e) mainly came from the plywood structural chitin fibers, both the SAXS and XRD results showed that the structure of chitin fiber remained unchanged during the demineralization process, while amorphous and crystalline mineral phases were removed. The 1D intensity profile I(Q) (Figure 5b) of GO shows a strong diffraction peak at 3 Å −1 corresponding to the (004) diffraction of GO, which contributing form the turbostratic structure of GO layers [35]. In addition, the intensity profiles I(Q) of CCF and CCF/rGO both show two distinctive peaks. The peak located at 1.3 Å −1 ((002) diffraction) indicates the interlayer structure in the materials. The (100) diffraction of CCF located at 2.4 Å −1 corresponds to the triperiodic order of the material [44]. The (100) diffraction of CCF/rGO slightly shifts to 2.3 Å −1 , which may be caused by the formation of a real graphitic phase. SAXS results of CCF and CCF/rGO are given in Figure 5e,f, respectively. The different colors in the SAXS patterns indicate the variations of the scattering intensities, and the maximum intensity is near the beam stop. Both CCF and CCF/rGO have a scattering pattern that looks similar to ellipse. This indicates the presence of the preferential oriented carbon fiber structure, which means the orientated fiber structure observed in CF was finely maintained after carbonization. The 2D integration profiles (Figure 5e,f) demonstrate that the oriented angle is about 100 degrees for both samples. The retaining of the plywood-style nanofiber arrangement is not only good for its mechanical function but also theoretically helpful in providing conductivity along different directions.

Properties Characterization
XPS analysis provides useful information about the surface elemental composition and functional groups. The C 1s XPS spectra of CCF and CCF/rGO before and after carbonization ( Figure 6) were collected to clarify the variations of the electronic state of carbon atom. The C 1s spectrum of CF (Figure 6a) is composed by three sub-peaks at 284.8, 286.1, 287.5 eV, corresponding to the C-C, C-O, and C=O bonds, respectively. The peaks for C-C and C-O bonds show similar intensities, which are stronger than that of C=O bonds. This result complies with the chemical composition of CF. After GO functionalization, the CF/GO shows a typical C 1s spectrum of GO (Figure 6b), indicating that the CF is totally coated by GO. After carbonization, both CCF and CCF/rGO show similar C 1s spectra (Figure 6c,d), mainly containing the C-C peak at 284.8 eV. Some other carbon atoms bearing oxygen-containing groups located at 286.1, 287.5 and 288.8 eV may have resulted from the partial decomposition of the polysaccharide structure [35,45].
The FTIR spectra of CCF and CCF/rGO are shown in Figure 7a. For both samples, the peaks at 2963 and 2877 cm −1 are attributed to the stretching vibration of CH 3 , and a peak at 1473 cm −1 belongs to the bending vibration of CH 3 . The peaks at 1383 and 739 cm −1 are, respectively, attributed to C-H and C-C. However, the peak at 1082 cm −1 attributed to the C-O-C skeleton vibration peak from aliphatic alkane structures. The results demonstrate the CCF and CCF/rGO are mainly composed of aliphatic alkane structures [46]. The structural properties of graphitic and non-graphitic carbon were further examined by Raman spectroscopy (Figure 7b). Two characteristic Raman bands for graphite can be found at 1330 (D band) and 1590 (G band) cm −1 in the obtained spectra. The D band is related to the structure defection and disorder of carbon, while the G band is corresponding to the crystallinity of graphite [47]. The relative intensity of the D band to the G band (I D /I G ) is usually used as a criterion to determine the degree of disorder for carbon materials. For both samples, the ratio of I D /I G were approximately 1, suggesting the formation of graphite structure after higher temperature treatment.   The FTIR spectra of CCF and CCF/rGO are shown in Figure 7a. For both samples, the peaks at 2963 and 2877 cm −1 are attributed to the stretching vibration of CH3, and a peak at 1473 cm −1 belongs to the bending vibration of CH3. The peaks at 1383 and 739 cm −1 are, respectively, attributed to C-H and C-C. However, the peak at 1082 cm −1 attributed to the C-O-C skeleton vibration peak from aliphatic alkane structures. The results demonstrate the CCF and CCF/rGO are mainly composed of aliphatic alkane structures [46]. The structural properties of graphitic and non-graphitic carbon were further examined by Raman spectroscopy (Figure 7b). Two characteristic Raman bands for graphite can be found at 1330 (D band) and 1590 (G band) cm −1 in the obtained spectra. The D band is related to the structure defection and disorder of carbon, while the G band is corresponding to the crystallinity of graphite [47]. The relative intensity of the D band to the G band (ID/IG) is usually used as a criterion to determine the degree of disorder for carbon materials. For both samples, the ratio of ID/IG were approximately 1, suggesting the formation of graphite structure after higher temperature treatment.
Further, the TGA curves in Figure 8a reveal the thermal behaviors of CF and CF/GO during carbonization. Both of them show a similar decomposition behavior, demonstrating the major weight loss occurring at the temperature range of 280-400 °C, as shown in Figure 8a, which corresponds to the decomposition of chitin. The depolymerization of chitin occurs simultaneously with dehydration and release of surface functional group. Further, at higher temperature, the transition process of pure sp3-carbon in a tetrahedral amorphous (ta-C) into a completely graphitic (sp2) one (g-C) might occur [31]. The weight yield of CCF/rGO (30.7%) at 800 °C is slightly higher than that of pure CCF (24.2%). This result is due to the flame-retarding behavior of GO in polymeric composites, which prevents the loose release of volatile products from degrading polymer [35]. The specific surface areas and pore-filling behavior of CCF and CCF/rGO were determined by nitrogen-adsorption isotherms. The BET surface area results confirms that both CCF and CCF/rGO composites have a mesoporous structure with a high surface area (Figure 8c,d). The isotherms exhibit an unclosed hysteresis loop with the desorption line under the adsorption line (inserts of Figure 8c,d). This phenomenon was previously observed in some carbonized high-molecular polymers and was ascribed to their complex "throat-or cavity-type" microporous structures [44]. Both isotherms belong to the type IV isotherm, confirming the mesoporous structure of the samples. Compared to the surface area of CCF (~76 m 2 /g), CCF/rGO shows a much higher value of ~187 m 2 /g. The pore-size distribution based on BJH analysis (Figure 8c,d) suggests the pore size of CCF decreases linearly in the range of 1.6-6 nm and centers at approximately 3.3 nm. In contrast, the pore size of CCF/rGO was distribution in the range of 2-42 nm, while the majority of pore size decreases linearly from 1.6-10 nm and centered at approximately 4.2 nm. Micropores (<2 Further, the TGA curves in Figure 8a reveal the thermal behaviors of CF and CF/GO during carbonization. Both of them show a similar decomposition behavior, demonstrating the major weight loss occurring at the temperature range of 280-400 • C, as shown in Figure 8a, which corresponds to the decomposition of chitin. The depolymerization of chitin occurs simultaneously with dehydration and release of surface functional group. Further, at higher temperature, the transition process of pure sp3-carbon in a tetrahedral amorphous (ta-C) into a completely graphitic (sp2) one (g-C) might occur [31]. The weight yield of CCF/rGO (30.7%) at 800 • C is slightly higher than that of pure CCF (24.2%). This result is due to the flame-retarding behavior of GO in polymeric composites, which prevents the loose release of volatile products from degrading polymer [35].

Conclusions
In summary, we have developed a novel carbon rich and porous self-supporting nanocomposites for advanced supercapacitor materials from the crustacean cuticle. The guiding principle is to make full use of the properties from its intrinsic interpenetrating nanofiber networks and porous canal system; this will further enhance its conductivity by carbonization and graphitization-all through simple and green material fabrication process. The cuticle was firstly demineralized to get a carbon rich freestanding scaffold composed of chitin-protein nanofibers, followed by utilizing the intrinsic hydrophilic properties of CF and GO, to attach them together and form a composite precursor. In the next stage, the simultaneous abilities of CF and GO to carbonize and to reduce to rGO, respectively, during high temperature treatment are used to obtain the final composite material. In this material, CCFs act as nano spacers in the interpenetrated CCF/rGO network, which enhances the supercapacitor's excellent volumetric electrochemical performance. A sustainable chitin-derived constituent makes a great ensemble with value-added graphene material. It is possible that further tuning of composite electrode materials based on biomass-derived nanocarbons and valuable additives is the future of cost-effective largescale production of energy storage devices.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. The specific surface areas and pore-filling behavior of CCF and CCF/rGO were determined by nitrogen-adsorption isotherms. The BET surface area results confirms that both CCF and CCF/rGO composites have a mesoporous structure with a high surface area (Figure 8c,d). The isotherms exhibit an unclosed hysteresis loop with the desorption line under the adsorption line (inserts of Figure 8c,d). This phenomenon was previously observed in some carbonized high-molecular polymers and was ascribed to their complex "throat-or cavity-type" microporous structures [44]. Both isotherms belong to the type IV isotherm, confirming the mesoporous structure of the samples. Compared to the surface area of CCF (~76 m 2 /g), CCF/rGO shows a much higher value of~187 m 2 /g. The pore-size distribution based on BJH analysis (Figure 8c,d) suggests the pore size of CCF decreases linearly in the range of 1.6-6 nm and centers at approximately 3.3 nm. In contrast, the pore size of CCF/rGO was distribution in the range of 2-42 nm, while the majority of pore size decreases linearly from 1.6-10 nm and centered at approximately 4.2 nm. Micropores (<2 nm) are observed in both samples with high proportion. Hierarchical micro/meso/macro-pores distribution can reduce the diffusion distance for electrolyte ions and decrease the electrical resistance of active biochar materials. Previous studies [13,16,45] have demonstrated that micropores (<2 nm) with ion accessibility can provide large capacitance. In addition, larger pores (2-50 nm) can offer channels for ion transport and reduce diffusion resistance, while macropores (>50 nm) help to form ion buffering reservoirs to decrease the ion transportation distance after electrolyte penetration. The synergistic effects of hierarchically porous structure ensure high affinity of charge transfer and faster ion diffusion process. Hence, the constructions of hierarchical electrodes enable unimpeded ion transportation and good capacitance of supercapacitors.
Carbonization of the CF and CF/GO lead to a strong fusion between the CCF scaffold and graphene sheets in the CCF/rGO composite. The well-distributed rGO sheets act as conductive connectors between CCF. The CCF/rGO composites show an electrical conductivity of 30.99 S/cm (Figure 8b), which is about 3 times higher than that (9.02 S/cm) of the pure CCF. To further evaluate the application potential of the developed composite as supercapacitors, the electrical capacitance of the CCF/rGO composites were characterized using two-electrode cells (described in the Supplementary Note S1). The composites exhibit comparable capacitance properties with previously reported materials [25][26][27].

Conclusions
In summary, we have developed a novel carbon rich and porous self-supporting nanocomposites for advanced supercapacitor materials from the crustacean cuticle. The guiding principle is to make full use of the properties from its intrinsic interpenetrating nanofiber networks and porous canal system; this will further enhance its conductivity by carbonization and graphitization-all through simple and green material fabrication process. The cuticle was firstly demineralized to get a carbon rich freestanding scaffold composed of chitin-protein nanofibers, followed by utilizing the intrinsic hydrophilic properties of CF and GO, to attach them together and form a composite precursor. In the next stage, the simultaneous abilities of CF and GO to carbonize and to reduce to rGO, respectively, during high temperature treatment are used to obtain the final composite material. In this material, CCFs act as nano spacers in the interpenetrated CCF/rGO network, which enhances the supercapacitor's excellent volumetric electrochemical performance. A sustainable chitinderived constituent makes a great ensemble with value-added graphene material. It is possible that further tuning of composite electrode materials based on biomass-derived nanocarbons and valuable additives is the future of cost-effective large-scale production of energy storage devices.  Data Availability Statement: All raw data that support the findings in this study are available from the corresponding authors upon request.